The present invention relates to aqueous adhesive compositions curable at room temperature and more particularly to an improved aqueous latex adhesive composition possessing improved water resistance and being especially adapted for use in structural lamination manufacture.
Various forms of plywood have found utility as structural or load-bearing components in the construction industry. While urea resins, melamine-urea cocondensation resins and phenol resins initially predominated as the adhesives of choice in plywood manufacture, aqueous-based adhesives have more recently found general acceptance in plywood manufacture. Aqueous adhesives can reduce the amount of organic volatiles released from the adhesive composition upon drying and generally possess less organic volatile material which can be released by the adhesive during use of the laminates over time. The reduction of water-sensitivity of the aqueous adhesives, however, was an initial concern of laminate manufacturers. This problem has been addressed by the art and several solutions proposed.
One solution germane for present purposes is the protected-isocyanate approach disclosed in U.S. Pat. No. 3,931,088. The water-resistant adhesive reported therein consists of an aqueous SBR or similar latex and an isocyanate dispersed in a hydrophobic solution of organic solvent. The latex, the surfactant associated with the latex, or an added ingredient bears active-hydrogen groups for linking with the isocyanate at room or elevated temperature. Another solution for present purposes is the emulsified isocyanate approach disclosed in U.S. Pat. No. 4,396,738. The water-resistant adhesive reported therein consists of an aqueous latex emulsion or dispersion of a vinyl polymer and a poly-isocyanate which has had some of its isocyanate groups reacted with a monohydroxy alcohol.
U.S. Pat. No. 3,996,154 and related British Pat. No. 1,444,933 disclose oil-in-water emulsions which are useful as adhesives. The emulsion comprises water, an aromatic polyisocyanate, and a non-ionic surface active agent which may be an alkyl ether of polyethylene glycol reacted with a polyisocyanate. Alternatively, the surface active agent may be the reaction product of a polyisocyanate and an ethoxylated alkanol such as shown in British Pat. No. 1,417,618. British Pat. No. 1,129,408 proposes to bond glass to rubber with an adhesive composition which is an aqueous dispersion or emulsion of a polyisocyanate which has been emulsified or dispersed with the aid of an emulsifying agent, a rubber latex, and optionally a polyepoxide compound. Emulsifying agents include non-ionic and anionic compounds, such as polyethylene oxide or polyvinyl ether. British Pat. No. 1,601,033 proposes a laminating adhesive which is an aqueous dispersion of a polyurethane latex and a hydrophobic polyisocyanate which is an aliphatic or cycloaliphatic polyisocyanate. Finally, U.S. Pat. No. 4,433,095 proposes an aqueous adhesive which is comprised of an aqueous polymer dispersion and a water-dispersible polyisocyanate which is made up of an aromatic polyisocyanate and an emulsifier produced by reacting aromatic polyisocyanates with hydrophilic compounds. Suitable hydrophilic compounds are shown to be monohydric or polyhydric, non-ionic polyalkylene polyether alcohols.